Typically, there are primarily three schemes as described below for realizing Extended Depth Of Field (hereinafter referred to as “EDOF”) in image-capturing devices.
The first scheme is a scheme in which an optical element, called a phase plate, is inserted, whereby the blur of the image is made uniform in the depth direction, and an image restoration process using a blur pattern obtained in advance through a measurement or simulation is performed, thus obtaining an image with an extended depth of field (hereinafter referred to as an “EDOF image”). This scheme is referred to as Wavefront Coding (hereinafter referred to as “WFC”) (Non-Patent Document No. 1).
The second scheme is a scheme in which a high-precision distance measurement is performed on each partial areas of an image by devising the diaphragm shape, and an image restoration process is performed by using a blur pattern obtained based on the distance of each partial area that has been measured in advance, thus obtaining an EDOF image. This scheme is referred to as Coded Aperture (hereinafter referred to as “CA”) (Non-Patent Document No. 2).
The third scheme is a scheme of moving the focus lens or the image-capturing element during the exposure time, convoluting an image that is uniformly focused in the depth direction (which is equivalent to making the blur uniform across different depths), and performing an image restoration process using a blur pattern obtained in advance through a measurement or simulation, thereby obtaining an EDOF image. This scheme is referred to as Flexible DOF (hereinafter referred to as “F-DOF”) (Non-Patent Document No. 3).
Another scheme known in the art is a scheme in which depth estimation or image sharpness detection is performed by utilizing the axial chromatic aberration of the lens, thereby obtaining an image that is sharp as a whole through image processes (Non-Patent Document No. 4). Also known in the art is a scheme in which the blur of an image is made uniform in the depth direction using a multi-focus lens, and an image restoration process is performed by using a blur pattern obtained in advance through a measurement or simulation (Non-Patent Document No. 5). However, these schemes, in principle, give a limited level of EDOF effects as compared with the three schemes described earlier.
Moreover, a scheme called “Focal Stack” has long been known in the art. In this scheme, a plurality of images of different focus positions are captured, and different areas believed to be in focus are extracted from the images and synthesized together, obtaining an EDOF image. This scheme requires many captured images, thereby requiring relatively a long time for capturing the images and consuming a large amount of memory space for storing the images.
With WFC, of the three schemes described above, various types of phase plates have been proposed, including Cubic Phase Mask (hereinafter referred to as “CPM”), Free-Form Phase Mask (hereinafter referred to as “FPM”), and the like, as those that give the highest level of EDOF effects. It is believed that FPM is prosperous in view of the image quality after restoration (how small the amount of artifacts is) (Non-Patent Document No. 6). However, a drawback common to all WFC schemes is that off-axis properties of the lens deteriorate by the insertion of a phase plate (Non-Patent Document No. 7). Specifically, since the same level of blur uniformizing effect cannot be realized for incident light coming from directions other than from the front direction, as compared with incident light from the front direction, the off-axis image quality after restoration deteriorate if the restoration process is performed using an axial blur pattern in the image restoration.
With CA, of the three schemes described above, due to the characteristic of the scheme itself that the distance measurement precision is enhanced by inserting a diaphragm of a characteristic shape, particular frequency components of a captured image or an image obtained following a restoration process are lost, i.e., the image quality deteriorates. Moreover, the amount of light is typically reduced from that of an ordinary image-capturing method, irrespective of the diaphragm shape, and it is therefore not suitable for capturing an image in dark places.
F-DOF, of the three schemes described above, is the scheme capable of producing the best image quality, and also gives a high level of EDOF effects. Since the off-axis characteristics are also dependent on the lens characteristics themselves, it is possible to easily enhance the performance. Note however that with F-DOF, an image of a good image quality is obtained when the same object is convoluted onto the same image position even if the position of the focus lens is moved during exposure. Therefore, an image-side telecentric lens may be used in some cases with F-DOF.
One field in which the EDOF technique is applied is microscopy. When capturing an image with a microscope, the object to be captured is a stationary object, and therefore it is possible to use some time to capture an image. Therefore, the focal stack scheme has long been used. Note however that since this scheme requires labor and time as described above, techniques have been proposed in which the F-DOF scheme is also used in combination (Patent Document Nos. 1 to 4). Where F-DOF is used in a microscope application, the sample, which is the object, or the lens barrel is moved during the exposure. Where it is assumed that a post-exposure image restoration process is performed, the object or the lens barrel is moved so that the blur of the image is always uniform. It has been known that appropriately controlling this manner of movement is practical because it is then possible to use an image restoration process method using a single blur pattern (Patent Document No. 5). For this purpose, when the image-capturing element is moved, the image-capturing element is moved at a uniform velocity. Where the focus lens is moved, it is necessary to realize focus displacement that is equivalent to the image-capturing surface moving at a uniform velocity (Non-Patent Document No. 3). It is known that the pattern of movement may be from the far-side focus end position to the near-side focus end position, or the opposite thereto.
In addition, examples have been known in recent years where the EDOF techniques are applied to cameras provided in mobile telephones, or the like. This is because due to the EDOF effects, an all-in-focus image (an image where all the objects are in focus) can be obtained without having an auto-focus mechanism. In this regard, of the three schemes described above, WFC and schemes using the axial chromatic aberration have been employed. F-DOF is not commonly employed because it requires a mechanism for moving the focus lens or the image-capturing element.